For precision machining of straight or convex surfaces, for example of radial bearings or control cams, eg of a crankshaft or camshaft or the like, there are known superfinishing machines in which, for example, a superfinishing stone or a grinding belt is placed against the point to be machined (DE-OS 41 21 18). In this case, both the superfinishing stone and the grinding belt are placed against the grinding position with a predetermined contact pressure. With superfinishing stones, it has proved disadvantageous that, especially in the case of workpieces having highly curved surfaces, unacceptably high degrees of deformation occur because of the large forces applied by the superfinishing stone to the workpiece. Furthermore, it has been found that, especially when machining cams, differing pressure conditions and differing machining speeds occur, due to the constantly altering curvature of the workpiece surface. This has the consequence that uniform machining over the periphery of the cam is not achieved.
When a grinding belt is used, these disadvantages can be reduced, at least within certain limits. Both when using a superfinishing stone and when using a grinding belt, the workpieces rotate and oscillate at the same time, so as to produce the desired cross-grinding. Because it is partially wrapped around the point for machining, the grinding belt does not, like a superfinishing stone, lie linearly on the curved surface for machining, but bears against the latter over a circumferential angle, albeit a very small one. However, the grinding belt is worn down very rapidly, since the grinding grains are detached from the adhesive composite after a very short time. The grinding action of this region of the grinding belt then falls off rapidly and becomes inadequate.
Moreover, it has been found that both a superfinishing stone and a grinding belt become smeared with dirt from the surface of the shaft being machined, with the result that the free spaces between the grinding grains are clogged and the cutting action becomes considerably reduced. As a rule, the grinding belt becomes unusable because of this soiling. When a grinding belt is used it therefore has to be transported on a certain distance so that a region of fresh, unused grinding belt is brought to the point being machined.
In addition, grinding belts have the advantage that they detach the ferrite particles which are enclosed, particularly, in cast material and round off the edges of the breakage points. Admittedly, superfinishing stones also detach these hard ferrite particles, but no rounding of the edges of the breakage points takes place, so that a piece remains. When the workpiece which has been machined in this manner is used later, this piece can break off and cause damage. The broken-off piece passes inevitably into the oil circulation. Grinding belts moreover have the advantage that they round off, for example, the edges of oil outlet bores, etc.
As already mentioned, the belt has to be transported on after a certain period of use, since the used region is either soiled or worn. For this purpose, belt feed devices are provided, which transport the belt on, either continuously or stepwise or incrementally. Such belt feed devices are either equipped with electric motors or are provided with a stepping feed mechanism, with the result that when a new shaft is inserted in the machining apparatus, in particular when the bearing to be machined or the surface to be machined is gripped in a tong-like manner, the grinding belt is transported on by a certain distance. However, stepping feed mechanisms of this type have the disadvantage that they do not transport the belt continuously during the grinding procedure, as is necessary especially when machining relatively large bearing points, since the grinding belt wears too quickly. Hitherto, this problem has been resolved by raising the grinding belt from the machining point during machining, transporting it the desired distance and then returning it. This is not only complicated and time-consuming but as a rule does not give the desired superfinishing result, since the material is not ground continuously.
Furthermore, as a rule electrical drives are wide enough for a plurality of belt-grinding stations to be arranged next to one another only with a large spacing. Using such apparatus, camshafts having cams and bearing points which lie close to one another, for example, cannot be machined in one operation, since the individual belt-grinding stations, because of their width, cannot be placed so close to one another that all the cams or bearing points can be machined simultaneously.